The present invention relates to a molding process for preparing porous articles of polytetrafluoroethylene (hereinafter referred to as "PTFE"). The porous PTFE articles obtained according to the present invention have specific characteristics due to their porosity, such as a permeability of gases and liquids, a property of catching and filtering fine particles, the lowest dielectric constant and dielectric loss tangent among solid materials, a toughness and a flexibility, in addition to own excellent properties of PTFE such as a heat resistance, a chemical resistance, a water and oil repelling property, an anti-tacking property, an incombustibility, a low friction factor and a weatherability. The molded articles are usable in wide industrial fields such as textile fields, medical fields as well as materials for filters and buildings.
Hitherto, porous PTFE molded articles have been prepared by paste-extruding non-baked PTFE fine powders obtained by emulsion polymerization together with a liquid lubricant, and then elongating (JP-B-13560/1967). However, this preparation process is suitable for preparing a wire coating, a thin sheet or a thin tube, but is not applicable to preparation of a thick article or a large article such as a cylindrical article.
The large and thick porous PTFE articles have been prepared by molding baked PTFE resin powders to a given shape under a pressure of 0.001 to 800 kg/cm.sup.2, and then baking it at a temperature of above the melting point of PTFE (JP-A-66730/1986). According to this process, however, since the hard PTFE powders are pressed under a relatively low pressure of 0.001 to 800 kg/cm.sup.2 into a given shape, the obtained pre-formed articles are brittle and difficult to handle. Further, for indending to prepare continuous porous PTFE articles when a continuous ram extrusion molding is employed, the bonding power between the pre-formed articles obtained by each stroke is poor. Therefore, continuous pre-formed articles cannot be prepared. For obtaining an additional bonding power, there is an atempt that a binder comprising a fluororesin powder or its dispersion is admixed. However, since an amount of the binder is at most 1% by weight, the continuous pre-formed article cannot be prepared by the above ram extrusion molding.
An object of the present invention is to provide a molding process for continuously, easily preparing large, thick and continuous porous PTFE articles.